The human body contains many fluids having vital functions within the body. For example, blood flowing in the circulatory system delivers necessary substances such as nutrients and oxygen to cells, and transports metabolic waste products away from those cells. Another fluid is the aqueous humor in the eyes. The aqueous humor maintains the intraocular pressure and inflates the globe of the eye, provides nutrition (e.g. amino acids and glucose) for the avascular ocular tissues, posterior cornea, trabecular meshwork, lens, and anterior vitreous.
Some properties of these bodily fluids are known to be indicative of a condition of the person's body, and determination of such properties may be used in order to monitor a person's health. For example, the blood glucose level (also referred to as blood glucose concentration) being too high or too low can be indicative of a malfunction of the digestive system, such as diabetes mellitus. Blood oxygen level is typically monitored to identify oxygen saturation condition that enables identification of hypoxemia as well allows estimation of hemoglobin in blood. Blood alcohol level (also referred to as blood alcohol concentration) is indicative of alcohol consumption and may be used to determine detrimental effects of alcohol on the gastrointestinal, cardiovascular and central nervous systems. Blood alcohol level is also indicative of impairment in a person's judgment and his ability to perform certain actions, such as driving a vehicle. In the eye, an important property of the aqueous humor is its pressure. This property is commonly called “intraocular pressure”. A high intraocular pressure may be indicative of disorders in the eye, such as glaucoma, iritis, and retinal detachment.
In the field of measuring blood-related parameters, such as glucose level and oxygen saturation, many non-invasive techniques have been devised, including impedance-based techniques and optical. For example, in glucose meters based on near infrared spectroscopy, a tissue is illuminated with light in the infrared spectrum, and the light reflected by the tissue and/or the light transmitted through the tissue is measured. The portion of light that is reflected and/or transmitted is indicative of the blood glucose level. Such glucose meters are used for tissue investigation in different depths varying from 1 to 100 millimeters or 10 to 50 micrometers. Some glucose meters use Raman spectroscopy to measure scattered light that has been influenced by the oscillation and rotation caused by glucose. Glucose meters based on photo-acoustic spectroscopy measure parameters of an acoustic pressure wave created by rapid heating of the sampled area. Other glucose meters measure changes in the scattering and the polarization parameters of light caused by glucose. Femtosecond pulse interferometry can be used to determine glucose concentration, by measuring the group refraction index of a glucose solution using a time delay of femtosecond order in a time-of-flight method. Optical coherence tomography can be used to measure and analyze the interference pattern between the coherently backscattered light from specific layers of tissues and a reference beam.
With regard to blood alcohol level, alcohol level is usually examined by determining blood alcohol concentration (BAC) in breath and blood of the affected person. The principle of BAC measurement is based on the fact that alcohol, taken orally, goes into the body system. Equilibrium distribution of alcohol into the different parts of the body mainly liver, kidney, brain, and lungs is attained very rapidly. The ratio of alcohol in the blood to alcohol in alveolar air is approximately 2,100:1 at 34° C., the temperature at which the breath leaves the mouth. Thus, the extent of alcohol intoxication or alcohol consumption is monitored by examining BAC in breath and blood of the affected person, but the obvious choice is blood, an absolute level can be obtained only by drawing a sample of blood. There are several methods for the estimation of BACs using iodometric titrations, breath analyzer, and biosensors.
With regard to intraocular pressure, the most commonly used ophthalmic device for measuring IOP, and current gold standard, is called applanation tonometer known as Goldmann tonometer. It is based on the assumption that the eye is a perfect sphere. Thus, the force required to achieve a fixed degree of applanation (3.06 mm in diameter) when the tonometer head directly applanates the cornea is converted into millimeters of mercury (mmHg) providing the IOP resisting this deformation. Despite of its accuracy and precision, Goldmann tonometry mainly suffers from inter-individual variations due to difference in corneal thickness and rigidity while being an invasive (contact) technique with limitations for monitoring the IOP over time. Note also that this standard method, which involves touching the cornea, also consequently necessitates the use of anesthetic eye drops. As alternative, one can measure the area of applanation when a given constant force is applied to the eye. This can be accomplished, for instance, by blowing from a given distance with a standard blast of air into the eye and measuring the applanation area of the cornea. Using this procedure, the contact in the measurement is avoided but the technique still remains unpractical for monitoring IOP at large periods of time, that is, it fails when identifying peaks and IOP variations.
This single measurement working principle of classical tonometers has encouraged researchers to develop new ways of continuous IOP monitoring. Some examples are the use of sensing contact lenses, some sort of implants with telemetric pressure transducers and devices based on optical principles. The latter is described for example in the following publications: Asejczyk-Widlicka, M., Pierscionek, B. K., Fluctuations in intraocular pressure and the potential effect on aberrations of the eye, Br. J. Ophthalmol. 91, 1054-1058, 2007; De la Torre-Ibarra, M. H., Ruiz, P. D., Huntley, J. M., Double-shot depth-resolved displacement field measurement using phase-contrast spectral optical coherence tomography, Opt. Express 14, 9643-9656, 2006; Matsumoto, T., Nagata, R., Saishin, M., Matsuda, T., Nakao, S., Measurement by holographic interferometry of the deformation of the eye accompanying changes in intraocular pressure, Appl. Opt. 17, 3538-3539, 1978.